Numerical analyses of the granite fragmentation in rotary-percussive drilling with the consideration of pre-existing cracks

Abstract

Rotary-percussive drilling technique is an efficient rock-breaking method and can be applied in geothermal drilling. The rock's fragmentation mechanism was investigated using the distinct element method with the consideration of pre-existing cracks. We developed a cutter-rock interaction model to simulate the rotary-percussive drilling. In this model, a discrete fracture network (DFN) was used to reflect the rock's complex behavior. Bonded Block Model (BBM) was used to simulate cracks initiation that can merge and propagate to fracture the rock. As a dynamic problem, the rock's damping effect was introduced into our model by fitting it to the laboratory tests. Results represented the rock's fragmentation process under the cutter indentation and the coupling of impact load and lateral movement well. The pre-existing cracks can significantly enhance the drilling speed, which demonstrates the significance of considering the rock's non-continuous nature. Raising dynamic impact load is good for improving the penetration results in our study. For the back rake angle, 50 is the best choice in the evaluated range. But the cutter with the back rake angle of 20 obtains the highest fragmentation volume and lowest specific energy under the coupling of impact load and lateral velocity. This paper's research is of great significance to guide the application of rotary-percussive drilling and reduce the cost of developing geothermal resources.